This invention relates to non-intrusive techniques for monitoring gas turbines. More particularly, the present invention relates to a method and apparatus for pro-actively monitoring the performance of a compressor by detecting precursors to a compressor surge event, and to determine and adjust the margin between the operating line of the compressor to its surge line.
The global market for efficient power generation equipment has been expanding at a rapid rate since the mid-1980xe2x80x2s. This trend is projected to continue in the future. The gas turbine combined-cycle power plant, consisting of a gas-turbine based topping cycle and a Rankine-based bottoming cycle, continues to be a preferred choice by power generation customers. This preference may be due to the relatively-low plant investment cost, the continuously-improving operating efficiency of the gas turbine based combined cycle, and the resulting favorable cost of electricity production using gas turbine combined cycle plants.
Elevated firing temperatures in the combustor of a gas turbine enable increases in combined cycle efficiency and specific output power. For a given firing temperature, an optimal cycle compressor pressure ratio exists which maximizes combined-cycle efficiency. This optimal cycle compressor pressure ratio is theoretically shown to increase with increasing combustor-firing temperature. Accordingly, there is a need for higher compressor pressure ratio in gas turbines due to the demands for increased power generation efficiency and increased combustor firing temperature.
In gas turbines used for power generation, a compressor preferably operates at a higher pressure-ratio to achieve a higher efficiency. During operation of a gas turbine, there may occur a phenomenon known as compressor stall and even surge, wherein the pressure-ratio of the compressor initially exceeds some critical value at a given speed, resulting in a rapid reduction of compressor pressure-ratio and airflow delivered to the combustor. Compressor stall results when the airflow separates from one or more compressor blades. Compressor surge results when the pressure ratio through the compressor becomes excessive and the airflow separates from all the compressor blades in one or more rows of a compressor. In surge, the compressor performance falls due to the inability of the compressor to handle the excessive pressure ratio. Compressor surge may result from a variety of reasons, such as, for example, when the compressor inlet profile of airflow pressure or temperature becomes unduly distorted during normal operation of the compressor. Compressor damage due to the ingestion of foreign objects or a malfunction of a portion of the engine control system may also result in compressor surge and subsequent compressor degradation.
Gas turbine compressors, including the axial compressors used in most industrial gas turbines, are subjected to demands for ever-increasing levels of pressure ratio, with the simultaneous goals of minimal parts count, operational simplicity, and low overall cost. Further, an axial flow compressor may be expected to operate at a heightened level of cycle pressure ratio at a compression efficiency that augments the overall cycle efficiency of a combined cycle power generation system that includes a gas turbine. An axial flow compressor is also expected to perform in an aerodynamically and aero-mechanically stable manner, i.e., to avoid a surge event, over a wide range in mass flow rate associated with the varying power output characteristics of the combined cycle operation.
Compressor surge is to be avoided. Compressor surge is an unstable oscillatory condition that reduces the mean airflow through the combustor. However, the need for high-pressure ratio and high efficiency compressor performance demands that gas turbine compressors be operated near surge conditions. The operating compressor pressure ratio of an industrial gas turbine is typically set at a pre-specified margin away from the surge boundary, generally referred to as surge margin, to avoid unstable compressor operation. In the past, surge margins have been static. The surge margin was established for a compressor and was not varied during compressor operation. Because the surge margin was static, the margin had to be set to avoid surge even for the worst case compressor conditions. However, the compressor generally did not operate in such worst-case conditions.
In the past, compressors have been restricted to operate in conditions that avoid surge by a wide surge margin. A maximum operating line has been established for each compressor that provides a wide margin between the compressor""s approved maximum operating conditions and the predicted surge conditions of a fleet average, i.e., not unit-specific.
The use of wide surge margins does not rely on sensing conditions that preceded surge. Surge margins are depended on the compressor speed, pressure ratio and flow rate. These conditions are not surge precursor conditions, but are general compressor operating conditions. To avoid surge and optimize performance, there is a long-felt need for systems that detect compressor conditions that precede surge, i.e., precursors to a surge event.
One approach to detecting a surge event is to monitor the air flow and pressure rise through the compressor. A range of values for the pressure rise is selected a-priori, beyond which the compressor operation is deemed to be unstable and the compressor operation is restricted to levels below the pre-selected range of values. In addition, rapid variations in the pressure rise across a compressor are monitored, as they also can be used to detect a surge event. Such pressure variations may be attributed to a number of causes such as, for example, unstable combustion, rotating stall, and surge events on the compressor itself. To determine these events, the magnitude and rate of change of pressure rise through the compressor are monitored. When such an event occurs, the magnitude of the pressure rise may drop sharply, and an algorithm monitoring the magnitude and its rate of change may acknowledge the event. This approach may detect a surge event that has already occurred. This approach, however, does not sense when surge is about to occur and does not provide a warning that the compressor is operating in conditions that are precursors to surge. This approach of identifying a surge event fails to offer prediction capabilities of rotating stall or surge event, and also fails to offer information to a real-time control system with sufficient lead time to issue surge avoidance actions, and thus fails to proactively deal to avoid a surge event.
The system disclosed here affords a method of compressor surge prediction, surge monitoring, and surge control that protects a compressor from surge damage, allows compressors to be operated with a reduced surge margin without actually incurring surge, allows for higher pressure ratios, and allows for improved compressor efficiency. This invention also improves the gas turbine power-plant combined-cycle efficiency. Simultaneous need for high cycle pressure ratio, high compressor efficiency, and ample (albeit reduced) surge margin throughout the operating range of a compressor is also addressed.
More particularly, the present system and method pro-actively monitor and control a compressor by identifying surge precursor conditions using a stall precursor detection algorithm and by sensing measurable conditions of the compressor. In an exemplary embodiment, at least one sensor is disposed about a compressor for measuring at least one compressor parameter. Such parameters may include, for example, air pressure, airflow velocity, and compressor vibration. Multiple sensors capable of measuring different compressor parameters may also be employed.
The sensors used are dependent on the particular implementation of the surge monitoring and prediction system. For example, some of the sensors sense dynamic pressure parameters like compressor pressure, and velocity of gases flowing through the compressor. Upon collecting a pre-specified amount of data from the sensors, the data is time series analyzed and processed to extract signal characteristics such as, for example, signal amplitude, rate of change, spectral content of the signal. The signal characteristics represent stall precursors. The stall precursors are used to determine near-surge conditions of the compressor.
The measured stall precursor values are then compared with corresponding characteristics for a similar compressor and also with average stall precursor characteristics computed for a plurality of similar compressors (referred to as xe2x80x9cfleet averagexe2x80x9d), and historical characteristics of the subject sensor. The compressor stall precursor characteristics are computed as a function of the underlying compressor operating parameters, such as, for example, pressure ratio, airflow, etc. and the comparison is used to estimate a degraded compressor operating map. The comparisons between the actual stall precursor values and the average stall precursor characteristics, or historical characteristics of the subject compressor yield a corresponding compressor operability measure. The compressor operability measure indicates whether the compressor is operating safely away from surge conditions.
Upon determining that the compressor is operating beyond a safe margin, for example, if the compressor is operating near surge conditions, then the real-time control system takes protective actions to mitigate risks to the compressor in order to maintain the required level of compressor operability
In one aspect, the invention is a method for monitoring and controlling a compressor, comprising (a) monitoring at least one compressor parameter; (b) storing the compressor parameter in a database system; (c) processing the monitored data using a stall precursor detection algorithm to determine a stall precursor; (d) comparing the stall precursor with at least one of a corresponding average compressor value, and a corresponding unit specific value to determine the level of compressor operability, and (e) if the stall precursor varies from at least one of the average compressor value and the unit specific value, performing corrective actions to vary the level of compressor operability to prevent a surge condition.
In another aspect, the invention is an apparatus for monitoring a compressor, comprising at least one sensor operatively coupled to the compressor for monitoring at least one compressor parameter; a processor system for computing stall precursors from the monitored data; a database system for storing the at least one compressor parameter and the computed stall precursors, the database system further comprising a look-up-table comprising average compressor values for similar compressors; a comparator for comparing the computed stall precursors with at least one of an average precursor value and a previously computed stall measure of the compressor; and a control system for initiating corrective actions to prevent a compressor surge if the computed stall precursors deviate from at least one of the average precursor value and the previously computed stall measure.
In yet another aspect, the invention is an apparatus for monitoring and controlling a compressor, comprising means for monitoring at least one compressor parameter; means for computing stall measures; means for comparing the stall measures with at least one of an average stall measure of similar compressors or a previously computed stall measure for the compressor; and means for initiating corrective actions if the computed stall measure deviates from at least one of the average stall measure or the previously computed stall measure.
In a further aspect, the invention is a method for detecting and compensating for a compressor surge in a gas turbine of the type having a compressor, the method comprising: remotely monitoring at least one compressor parameter; computing stall measures; comparing the stall measures with at least one of an average stall measure of similar compressors or a previously computed stall measure for the compressor; and initiating corrective actions if the computed stall measure deviates from at least one of the average stall measure or the previously computed stall measure.
In yet another aspect, the invention is a method for remotely monitoring and controlling a compressor to prevent a compressor surge condition, the method comprising: (a) monitoring at least one compressor parameter and storing data indicative of the monitored parameter in a remote database system; (b) processing the monitored data using a stall precursor detection algorithm to determine a stall precursor; (c) comparing the stall precursor with at least one of a corresponding average compressor value, and a corresponding unit specific value to determine the level of compressor operability, and (d) if the stall precursor varies from at least one of the average compressor value and the unit specific value, performing corrective actions to vary the level of compressor operability to prevent a surge condition.